Sunday, 30 October 2016

Out of Tibet - Origins of the First Sheep

The argali is a species of wild sheep still found in Tibet

Sheep, as I have noted before, are members of the goat subfamily within the much larger group of the bovids. They are, in essence, a special kind of goat that has adapted to living in the foothills of higher mountains, rather than on the steeps-sided peaks themselves. The exact number of species of sheep is debatable, but there are at least five wild ones, plus the domestic animal, for a total of six.

When I discussed the evolutionary history of the goat subfamily a few years back, I talked primarily about the goats themselves, and about the muskoxen, and said very little about how the sheep became separated from the goats, and how they evolved since. A new paper, however, goes into much more detail about the origin of sheep and their fossil history. It is, in fairness, based on just one pair of fossilised horns, but, nonetheless, this is as good a starting point as any, and a sound excuse for me to summarise what we do know about sheep evolution.
Taking our definition of "sheep" to be "any member of the genus Ovis, or any animal more closely related to that genus than to any other animal alive today", there are relatively few sheep fossils known. (It should, of course, be noted that this definition is somewhat arbitrary, since we'd first need to define what the genus Ovis is...) Those fossils we do have come primarily from northern Asia and from Europe, with a few particularly recent ones from North America. They are all Pleistocene in age, although they don't necessarily all belong to modern species, with at least five fossil species having been named - most of them in the 19th century.

With relatively little in the way of fossils to go on, we are left to infer relationships based on the genetic differences between living species, calibrated against the known specimens we do have to arrive at a typical mutation rate over time. Doing so implies that sheep have been around a very long time, perhaps 10 million years or so, and certainly well before any of the known fossils. Such analyses also show a split within the sheep group, whereby one group headed into north-east Asia, and then crossed the land-bridge to North America, while the other initially remained in Asia, and later also reached Europe. The first group leads to bighorn sheep, snow sheep, and Dall's sheep, while the other is the ancestor of all remaining modern species, including, eventually, the domesticated one.

This implies that sheep originated somewhere in Asia, especially since so few of the fossils we have (and none of those that are at all old by the standards of such things) come from anywhere else. Since sheep likely evolved somewhere mountainous, Tibet seems a likely starting point for looking for that origin. This is also where the argali species of wild sheep lives today, and very close to the ancestral home of the urial, the animal from which domestic sheep were first bred in the aftermath of the last Ice Age.

The problem is, we don't actually have any fossil sheep from central Asia.

And this is where the new paper comes in, because the authors think they've found one. But, then, we do know of some fossil goats (at least in a broad sense) from the area, so how do we know that the specimen they're looking at is, in fact, a sheep, not a goat? After all, all they have are its horns, and they were initially identified as belonging to a relative of the living blue sheep (Pseudois spp.), a goat-relative found in places such as Nepal.

Looking across all the species of the goat subfamily, most animals have short horns, which tend to be relatively narrow, and mostly straight, with perhaps a slight curvature towards the back. This, for example, is what the horns of chamois look like. It's most likely the ancestral pattern, but, in some caprine lineages, the shape has become altered, and more impressive. Specifically, in the case of sheep, what we see are the tight curls of "ram's horns", rather than straight prongs, or the much wider arcs of animals such as ibex and wild goats.

The problem there, however, is that horns are composed of two main layers. The outer one, which we see in living specimens, is made of dense keratin - the substance actually known as "horn". But this decays after death, and doesn't typically survive fossilisation. It might if the animal fell into something like an undisturbed de-oxygenated mud field or the like as it died, as evidenced by those few fossils that preserve remarkable details of soft body anatomy.. but that frankly isn't very likely for an animal that lives half way up a mountain. So, what we actually have are the horn cores, the bony supports that form the inner layer of these structures.

The modern species of sheep; Protovis would branch off
from the bottom

The horn cores analysed in the paper, belonging to an animal that lived 3 to 5 million years ago in what is now Tibet, arch upwards and curve out to the side. They don't exactly resemble either the horn cores of modern true sheep, or of blue sheep, so that the authors assign a new genus name to the fossil, which accordingly has the full name Protovis himalayensis. In some respects, they are between the two, with more of a curve than one would expect from Pseudois (the blue sheep), and less than one would expect from Ovis. But they are flat along one edge and curve to the rear, as is the case for sheep, and the internal structure of the cores, which contain struts that help to absorb impacts during head-butting, are also said to be closer to those of Ovis than to any of its relatives.

That the horns are shorter, slimmer, and less curved than those of living rams would suggest that this is a more primitive form, which would make sense, considering its age. It's plausibly a close relative of the first Ovis sheep, existing before the eastern and western species of the genus went on their separate evolutionary paths. It's also consistent with - although not, to my mind, water-tight evidence of - an origin for the sheep clade somewhere in the Tibetan Plateau.

If so, it would hardly be alone; many other groups of animals, such as woolly rhinos, big cats, arctic foxes, and early dholes all seem to have had their origin in Tibet, spreading out around the world as the Ice Ages spread, perhaps making their original homeland too inhospitable even for them. That sheep may also have been among this list is not particularly surprising, suggesting that the harsh conditions of the place induced a more rapid, or at least more dramatic, evolutionary shift among their ancestors than among the early chamois and serows, which have changed relatively little from their assumed ancestral form.

Since all we have are the horns, its hard to say too much about what Protovis would have been like when it was alive. It presumably ate grass, since, even that far back, there isn't much else that it could have eaten in large quantities on the Plateau. It evidently head-butted its rivals, and presumably in the style that sheep do today, given the shape and structure of the horns. Indeed, the size of the horns of this particular specimen suggest that they must have belonged to a male, even if they aren't quite what we'd expect on a modern ram.

Given the locality, it must have lived at high altitude. The climate at the time it lived was more mild than today's, but, even so, being so far up a mountain side must have surely equipped the animal for a harsh habitat. When the Ice Ages arrived, these early sheep - perhaps by now evolved to a point that we could call them early Ovis - came down from the mountains, and found that the lowlands now suited them far better than they had before. From here, they spread out, reaching Europe and North Africa in one direction, and North America, by way of Siberia, in the other.

At least some of their descendants, retreating back to whatever hills and low mountain ranges that they could find in their new homes, survived the end of the Ice Ages, and the expansion of humanity, long enough to eventually become domesticated... and far too valuable to be driven extinct as so many of their former contemporaries were.[Photo by "Momotarou2012", from Wikimedia Commons. Cladogram adapted from Wang et al. 2016.]